process.connected
如果Node.js进程是由IPC channel方式创建的(请看[Child Process子进程][] 和 [Cluster集群][] 文档)，
只要IPC channel保持连接， process.connected属性就会返回true。
process.disconnect()被调用后，此属性会返回false。
process.connected如果为false，则不能通过IPC channel使用process.send()发送信息。
If the Node.js process is spawned with an IPC channel (see the Child Process
and Cluster documentation), the process.connected property will return
true so long as the IPC channel is connected and will return false after
process.disconnect() is called.
Once process.connected is false, it is no longer possible to send messages
over the IPC channel using process.send().